The use of colloidal dispersions is well known in the art of coatings technology. Such dispersions, when incorporated into coatings, are known to improve mar and scratch resistance. They are also known to improve the storage stability of the coating compositions, to assist in rheology control of coatings during application to a substrate, and to improve orientation of pigment particles in coatings containing metallic and other effect pigments.
The favorable effects of the colloidal particles are due in large part to the very small size of the dispersed particles, which is less than the wavelength of visible light. This very small particle size prevents the particles from scattering light, thereby preventing haziness and adverse color effects that can occur from light scattering in an applied coating. The small particle size also promotes stability of the colloidal dispersions as well as the stability of the coating compositions that contain such dispersions. Ambient forces in the suspension such as electrostatic repulsion and Brownian motion of the suspending medium are sufficient to overcome the effects of gravity on the particles. The particles can remain in suspension indefinitely under normal . conditions.
It is well known in the art that very small particles, in particular silica particles, can associate with one another, forming agglomerates. In coatings, these agglomerates act essentially as large particles, and therefore, the above-mentioned benefits of the small particle size may be lost. This tendency to agglomerate is believed to be due to the presence of polar moieties on the surface of the particles, for example Si—O—H moieties on silica particles, which can associate with the surfaces of adjacent particles. One way of preventing this interaction of polar groups on adjacent particles is to form the particles, or later suspend the particles, in an aqueous carrier. Water molecules in an aqueous carrier successfully compete with the neighboring particles for interaction with the polar groups. Although the stability of the suspension can be affected by factors such as pH and the presence of cations, particularly polyvalent cations, the incorporation of aqueous dispersions into aqueous. coating compositions is relatively straightforward. However, in organic coatings or coatings with a high level of non-polar components, the particles have an increased tendency to agglomerate. Since many coating compositions are solventborne, it is desirable to provide a means of incorporating these colloidal dispersions of particles without agglomeration of the particles.
A number of approaches to preventing this agglomeration are known in the art. One approach is the reaction of the silica surface with silanes or silylating agents to yield silica powders dispersible in organic media. Also known is a method in which a non-polar organic solvent, cationic surface active agent, and an organic group-substituted silane are added to an aqueous dispersion of colloidal silica and the water removed by azeotropic distillation.
Another approach involves association of the polar groups of the silica particles with other polar or functional groups to prevent interaction of polar groups on adjacent silica particles. Such approaches include associating the polar moieties on the surface of the silica particles with low molecular weight alcohols such as methanol or propanol. The association may be through polar attraction or hydrogen bonding, or through the formation of chemical bonds.
Preparation of colloidal silica dispersions in solvents such as dipropyleneglycol monomethyl ether is known. In this method, a colloidal dispersion of silica particles is added slowly to the solvent while water is distilled off under vacuum at relatively low temperatures. A disadvantage of stabilization of silica particles by solvent alone is that when the colloidal silica is added to a coating composition and the coating composition is subsequently applied to a substrate and dried, some or all of the stabilizing solvent is evaporated. This may lead to destabilization of the silica suspension, haziness, and loss of other coating properties such as optimal appearance and mar resistance. Accordingly, it is desirable to provide a method to stabilize colloidal particles in such a way that they remain stably dispersed throughout the drying and curing of a coating.